drivers: i2c: i2c_dw: add bus mutex

This change adds a mutex to protect against simultaneous access to the bus
instead of returning an error during transfers. Since most I2C code doesn't
handle retries (especially with a -EIO code) not blocking on a mutex can
cause a number of problems.

Signed-off-by: Corey Wharton <xodus7@cwharton.com>